This invention relates to the rejuvenation or reactivation of deactivated automobile emission control catalysts. More particularly, it relates to the rejuvenation of such catalysts by at least a partial removal of lead, sulfur and phosphorus compounds from the surfaces thereof, and by increasing the surface area of the active metal or metals contained therein.
Much research has been conducted to discover a method to reduce the air pollutants in the exhaust gas of automobile engines. At the present time it is thought that the most feasible method involves, or will involve, the utilization of some form of catalytic converter. This device generally operates in two stages, the first to reduce NO.sub.x compounds to nitrogen, and the second to oxidize CO and hydrocarbons.
Several combinations of catalysts are known in the art which can achieve the desired activity and selectivity for NO.sub.x conversion in the first stage and for the oxidation of CO and hydrocarbon gases in the second. Despite their relatively high cost, preferred catalysts comprise rhodium, with or without added nickel, on bases of alumina or aluminum borate for the NO.sub.x reduction stage, and platinum and/or palladium on similar bases for the CO/hydrocarbon oxidation catalyst. When exposed to exhaust gases containing even very minor amounts of compounds of lead and/or phosphorus these catalysts eventually become deactivated due to contaminant poisoning. Thus usually occurs after about 20,000 miles of engine operation, or sooner if the engine is not operated exclusively on Federally Certified gasoline (less than 0.05 g Pb and less than 0.005 g P per gallon).
Research into the nature of the contaminant poisons reveals that the chemical compounds responsible for catalyst deactivation are derived from the compounds of Pb, P and S present in the gasoline, P, S, Zn Ca, Mg and Ba present in motor oil and Fe, Cu, Cr and Ni present in the metals composing the engine. The catalytic poisons other than compounds of P, S and Pb generally deactivate the catalysts only when present in large amounts, the primary mechanism of their deactivation being inhibition of diffusion of the exhaust gases to the catalyst surface and shielding of the active catalyst sites. Lead, sulfur and phosphorus, which deposit mainly as the compounds, PbO, PbO.sub.2, nPbO.PbCl.sub.2, nPbO.PbBr.sub.2, 3Pb.sub.3 (PO.sub.4).sub.2.PbCl.sub.2 (or Br.sub.2), PbO.Pb.sub.3 (PO.sub.4).sub.2, PbSO.sub.4, PbCl.sub.2, PbBr.sub.2 nPbO.PbSO.sub.4 and Pb.sub.3 (PO.sub.4).sub.2 are known rapidly to deactivate the noble metal catalysts when as little as 0.5 to 8 percent by weight, determined as elemental P and/or Pb, of such compounds deposit thereon. A major problem confronting industry lies in developing effective and economical methods for rejuvenating such poisoned catalysts. To date, no such method has been developed.
It has now been found that the contaminant poisons previously discussed, especially lead sulfate, can be removed from NO.sub.x reduction and CO/hydrocarbon oxidation catalysts, and the same effectively regenerated thereby, by the three-step process of subjecting such catalysts to: (1) extraction with a concentrated aqueous solution of ammonium salt, (2) sulfation in an atmosphere containing SO.sub.2 or SO.sub.2 and oxygen, and (3) extraction once again with an aqueous ammonium salt solution. The purpose of the first extraction step is mainly to dissolve out lead sulfate; that of the sulfation step is to convert the other lead compounds into lead sulfate so that they may be dissolved in like manner in the second extraction step. Phosphorus is partially removed in the sulfation step by volatilization to gaseous P.sub.2 O.sub.5. Other contaminant poisons such as Fe, Zn, Mg, etc., are also found to be removable by the process of the invention. Catalytic rejuvenation in the manner described herein recovers between 50 and 75 percent or more of the original fresh activity lost by contaminant poisoning.
A somewhat puzzling aspect of the invention is that the proportion of lead and phosphorus compounds removed from the catalysts by identical rejuvenation treatments varies greatly from catalyst to catalyst. However, even in cases where as little as 2-10% of the lead and 5-20% of the phosphorus are removed, there is still a substantial rejuvenation in activity. It would hence appear that (1) only a small proportion of the total lead and phosphorus deposits on the catalysts are associated with the active metal or metals in a deactivating relationship, the remainder being distributed in relatively harmless locations, and (2) the small deactivating proportions are fortunately the most readily removable by the rejuvenation treatment. It is also possible that in some cases the rejuvenation treatment may bring about a desirable redispersion of agglomerated active metals.